The commercial photovoltaic market is dominated by crystalline silicon wafer technology. However an increasing share of the market is shifting toward thin-film materials and/or a thin wafer format for crystalline materials including silicon because of anticipated cost reductions. The handling of such thin solar absorbing layers, whether amorphous or crystalline, involves the use substrates.
Conventional solar cell devices that rely on a thin solar absorbing material comprise on the sunny side an active layer, a passivation layer(s) on the active layer, an anti-reflective (AR) coating or a TCO electrode on the passivation layer(s), metal contacts on the anti-reflective coating or TCO, and an optically transparent (transparent) substrate (e.g., glass). The back side comprises the active layer on a bottom electrode, the active layer with an intermediate passivation layer on a bottom electrode, the active layer on a seed layer, or the active layer on a TCO and a transparent substrate. The TCO can comprise indium tin (ITO), aluminum doped zinc oxides (AZO), boron doped zinc oxide (BZO), fluoride doped tin oxide (FTO), or Fluorine doped Zinc Oxide (FZO), which are the most commonly used transparent conductor materials. Sunlight (photons) is received on the glass substrate side of the solar cell, and is transmitted through the TCO to the active layer which generally comprises silicon. The TCO can provide greater than 80% transmission over a wavelength range from about 250 nm to 1.1 μm.
Electron-hole pairs are generated in the active layer by incident photons having sufficient energy relative to the band gap energy from the sunlight. Individual solar cells, which generate a relatively low voltage (typically 0.5 to 0.6 volts), are combined in series to provide higher output voltages. The efficiency of silicon-based solar cells is known to be improved by thinning the active layer because of the resulting higher open-circuit voltage. Conventional silicon solar cells are currently limited to about 19% efficiency for multicrystalline silicon and 21% conversion efficiency for monocrystalline silicon. Silicon solar cells based on advanced architectures such as the interdigitated all back contact (IBC) solar cell currently achieve efficiencies around 24%.
Next generation passivated emitter and rear contact (PERC)/Selective Emitter solar cells are anticipated to improve the efficiency of conventional silicon solar cells from 20% to 22% efficiency while Heterojunction Intrinsic Thin Layer (HIT) (efficiency currently about 25%) and the IBC (efficiency currently 24%) cells are expected to improve and achieve efficiencies well beyond 25% in the future. The HIT solar cell comprises thin single crystalline silicon surrounded by ultra-thin amorphous silicon layers.
Conventional processing to form HIT solar cells begins with a clean crystalline silicon (c-Si) wafer surface prepared before the amorphous silicon (a-Si) deposition, which generally includes an HF dip for surface silicon oxide removal. Next, an intrinsic amorphous silicon (i-a-Si) layer (passivation layer) followed by a p-type amorphous silicon (p-a-Si) layer is deposited in a high vacuum on one side of the wafer to form the p/n heterojunction. The step is repeated for the back side of the wafer where an intrinsic amorphous silicon (i-a-Si) layer (passivation layer) is deposited followed by an n-type amorphous silicon n-a-Si layer to obtain a Back Surface Field (BSF) structure. Subsequently, TCO layers are formed on both sides and on top of the n/p amorphous layers, and finally, metal grid electrodes are formed also on both sides of the wafer using a screen-printing method.
The heterojunction solar cell includes an amorphous semiconductor deposited onto a crystalline semiconductor to create passivation and form a heterojunction. A typical heterojunction solar device (e.g. HIT cell) includes deposited intrinsic (passivation layer) and doped amorphous (a)-Si layers on top of a crystalline c-Si layer forming a heterojunction front emitter and a back surface field in the rear contact.